1. Field of the Invention
This invention relates to layered semiconductor structures and more particularly, to layered III-V semiconductor structures with high nitrogen contents and to light emitting devices including the layered structures for use in laser diodes, photoreceptors and other similar devices.
2. Description of the Related Art
Compound semiconductors in general and AlGaInP semiconductor materials in particular have recently received considerable attention as a group of new semiconductor materials. These materials are well suited for the use of high intensity light sources such as, for example, light emitting diodes for color display devices, which have light emissions ranging from green to red, and semiconductor laser diodes in the visible wavelength region for optical recording and printing systems.
The AlGaInP semiconductor materials have a largest band gap energy among III-V alloy semiconductors of the direct transition type, which are lattice-matched to GaAs. A maximum band gap energy for the materials reaches approximately 2.3 eV or 540 nm in wavelength.
To construct a light emitting device, a heterojunction structure is formed, which basically comprises a narrow band gap active (light emitting) layer joined to a relatively wider bandgap, lattice-matched cladding layer.
When a heterojunction is formed using the AlGaInP materials, a relatively small conduction band discontinuity (xcex94Ec) results, in general, between the active and cladding layers. This small band discontinuity causes injected carriers (or electrons) to overflow from the active layer to the cladding layer with relative ease, thereby giving rise to disadvantages such as, for example, a large variation of a laser threshold current density with temperature, and unsatisfactory temperature characteristics of the light emitting devices constructed with the materials.
To achieve a satisfactory carrier confinement, thereby overcoming the above-mentioned difficulty, a structure has been disclosed in Japanese Laid-Open Patent Application No. 4-114486 (1992), in which a multi-quantum barrier (MQB) structure is provided between an active layer and a cladding layer, though this results in a more complicated structure.
In order to achieve laser emission, it is essential to attain the confinement of carriers and light beams into an active layer which is sandwiched between cladding layers to form a double heterostructure (DH). Although an active layer material having a relatively large band gap energy is required to achieve laser emissions at shorter visible range wavelengths, the band gap energy for the material can not be too large because of the relative magnitude of band gap energies described just above, as long as the material is used in bulk for forming the DH structure.
As an example, continuous laser emissions at 632.7 nm at room temperature is described using an (Al0.19Ga0.81)0.5In0.5P active layer by K. Kobayashi and others, Japanese Journal of Applied Physics, Vol. 29, page L 1669 (1990). Further, to attain laser emissions at shorter wavelengths, a quantum well (QW) structure has been developed. In addition to achieve a low lasing threshold current, a strained QW structure including strained quantum well layers has been proposed in Japanese Laid-Open Patent Application No. 6-77592.
Also, continuous laser emissions at 615 nm at room temperature have been described by H. Hamada, Electronic Letters, Vol. 128, p 1834 (1992). This laser device includes an (Al0.08Ga0.92)0.45In0.55P quantum well layer incorporated into compressively strained multi-quantum well active layers combined with multi-quantum well barrier (MQB) structure. The device is of almost no practical use, however, due to its unsatisfactory temperature characteristics.
Furthermore, to fabricate a laser diode on a silicon or GaP substrate, nitrogen-containing III-V alloy semiconductors such as InNSb and AlNSb, are disclosed in Japanese Laid-Open Patent Application No. 7-7223 (1995). In that disclosure, the band gap energies of the two semiconductors, InNSb and AlNSb, are estimated by linearly interpolating band gap energies of InN and InSb, and AlN and AlSb, respectively, to find that Al N0.4Sb0.6 is lattice-matched to GaAs, and that has a band gap energy of about 4.0 eV.
If the above alloy semiconductor is feasible, light emitting devices may be fabricated, which have emission wavelengths ranging to the ultraviolet spectral region. However, since almost all of these nitrogen-containing alloy semiconductors are in the non-miscible region in the solid solubility diagram, they are not feasible by conventional crystal growth methods but only by non-equilibrium growth methods such as, for example, metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).
However, even by MOCVD and MBE, the nitrogen content has not been able to exceed 10%, and the content of about 40% which is considered to be preferable to the device application, is quite difficult to achieve. In addition, as disclosed in Japanese Laid-Open Patent Application No. 6-334168 (1994), a relatively large degree of the energy level bowing is present owning to a large electronegativity of nitrogen. Therefore, band gap energies of these materials decrease by adding more nitrogen into InSb or AlSb, and at the alloy composition for which the lattice-matching to GaAs or Si is attained, the band gap energy is smaller than those of InSb or AlSb in contrast with the above expectation.
Accordingly, it is difficult to form an alloy semiconductor such as disclosed in the above Patent Application No. 6-37355. By utilizing the energy band bowing, on the other hands, a light emitting device with 1.5 micron emissions may be achieved With a GaInNAs material formed on a GaAs substrate, as described in Japanese Laid-Open Patent Application No. 6-37355 (1994).
In prior device fabrication methods, the growth of GaInNAs layers having N as a group-V element were carried out by simultaneously supplying each of source materials for Ga, In, N and As to achieve a constant composition throughout the thickness of the alloy layers.
However, in such GaInNAs semiconductor alloy system, the alloy layers are generally grown with a mixing ratio of the third additive element of only a few percent different than the stoichiometric compositions.
In addition, a monatomic superlattice structure has been disclosed in Japanese Laid-Open Patent Application No. 7-263744, to grow a semiconductor alloy having an N content higher than those formed by previous growth techniques. The monatomic superlattice structure in the disclosure comprises a systematically layered structure with a first monatomic layer including one of the group-III elements and one of the group-V elements other than N, and a second monatomic layer including one of the group-III elements and N as the group-V element.
For example, a xe2x80x9cunit structurexe2x80x9d is first constructed from eight monatomic layers, in which six GaP first monatomic layers and two GaN second monatomic layers are deposited in a predetermined order. Second, by systematically depositing a plurality of the unit structures, a light emitting layer is formed.
This disclosure states that a superlattice structure can be formed having a bandgap energy approximately the same as that of a GaNP mixed crystal, and that mixed crystals of GaNP and similar crystals can be formed having higher N contents, which have not been achieved through prior growth techniques.
Since some of the monatomic layers of III-V semiconductor compound are used including only N as the group-V element in the above disclosure in the Patent Application ""744, layered mixed crystal structures may be formed having high N contents and predetermined compositions with an N content of 12.5, 25, 37.5, 50, 62.5, 75 or 75.5 percent.
However, there is a certain range of composition, which may not be achieved by this method. For example, in order to obtain an N content as low as 1%, a structure has to be constructed from 99 monatomic layers without N as the group-V elements and one monatomic layer containing N. This results in a portion of the structure which is composed only of the III-V compound without any of N content over a thickness of approximately 30 nanometers. This results in properties similar to bulk semiconductor materials rather than those of the desired monatomic superlattice structure having a single average composition. Therefore, the above-mentioned method is not satisfactory for growing semiconductor alloys having an arbitrary elemental composition.
Accordingly, it is an object of the present invention to provide a layered structure of a III-V semiconductor alloy including N and at least one of the other group-V elements and a plurality of the group-III elements, capable of having a deduced average composition which has been difficult to achieve by the previous growth method, and having both a conduction band offset AEc and a valence band offset xcex94Ev large enough for the practical applications to light emitting devices such as laser diodes, for example.
It is another object of the invention to provide a layered structure of a III-V semiconductor alloy including N and at least one of the other group-V elements and a plurality of the group-III elements, capable of providing light emitting devices such as red wavelength laser diodes with excellent temperature characteristics, visible wavelength laser diodes which may achieve emissions shorter wavelengths than 600 nm, and visible region light emitting diodes having a high intensity.
According to one aspect of the invention, the III-V semiconductor alloy layered structure comprises first and second monatomic layers, in which the first monatomic layer including a III-V semiconductor alloy containing N, at least one other group-V element, and at least one group-III element; and the second monatomic layer includes a III-V semiconductor alloy containing no N and at least one group-V element excepting N, and at least one group-III element. Through the deposition of the first and second monatomic layers in a predetermined order, the III-V semiconductor alloy structure is formed as a superlattice structure having a deduced average composition.
According to another aspect of the invention, the III-V semiconductor alloy structure is formed with first and second monatomic layers so as to have a reduced composition of GaxIn1xe2x88x92xNyAs1xe2x88x92y, where 0 less than x less than 1 and 0 less than y less than 1.
The first monatomic layer has an N content and a composition of Gax1In1xe2x88x92x1NyAs1xe2x88x92y, where 0 less than x1xe2x89xa61 and 0 less than y less than 1, and second monatomic layer has no N content and has a composition of Gax2In1xe2x88x92x2As, where 0xe2x89xa6x2 less than x1xe2x89xa61, respectively. In addition, at least one each of the first and second monatomic layers is deposited in a predetermined order to form the III-V semiconductor alloy structure as a superlattice structure.
According to another aspect of the invention, the III-V semiconductor alloy structure is formed with first and second monatomic layers so as to have a reduced composition of (AlxGa1xe2x88x92x)yIn1xe2x88x92yNzP1xe2x88x92z, where 0 less than xxe2x89xa61, 0 less than yxe2x89xa61 and 0 less than z less than 1.
The first monatomic layer has an N content, no Al content and a composition of GayIn1xe2x88x92yNzP1xe2x88x92z, where 0xe2x89xa6yxe2x89xa61 and 0 less than z less than 1, and second monatomic layer has no N content, an Al content and a composition of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP, where 0 less than xxe2x89xa61 and 0 less than yxe2x89xa61, respectively. In addition, at least one each of the first and second monatomic layers is deposited in a predetermined order to form the III-V semiconductor alloy structure as a superlattice structure.
According to another aspect of the invention, the III-V semiconductor alloy structure is formed with a monatomic layer which has neither N nor Al content and a composition of GayIn1xe2x88x92yP (0xe2x89xa6yxe2x89xa61), is interposed between first and second monatomic layers.
The first monatomic layer has an N content and a composition of GayIn1xe2x88x92yNzP1xe2x88x92z (0xe2x89xa6yxe2x89xa61 and 0 less than z less than 1) and the second monatomic layer has no N content, an Al content and a composition of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0 less than xxe2x89xa61 and 0 less than yxe2x89xa61).
According to another aspect of the invention, light emitting devices are constructed, incorporating the III-V semiconductor alloy structure as an active layer, in which the III-V semiconductor alloy structure is as specified in the above embodiment 1, 2, 3 or 4.